This application claims priority to an application entitled xe2x80x9cPower Control Device and Method for Reverse Link Common Channel in Mobile Communication Systemxe2x80x9d filed in the Korean Industrial Property Office on Jul. 13, 1998 and assigned Ser. No. 98-28238, and an application xe2x80x9cPower Control Device and Method for Reverse Link Common Channel in Mobile Communication Systemxe2x80x9d filed in the Korean Industrial Property Office on Aug. 5, 1998 and assigned Ser. No. 98-32351, the contents of which are hereby incorporated by reference.
1. Field of the Invention
The present invention relates generally to a power control device and method for a mobile communication system, and in particular, to a power control device and method for a reverse link common channel.
2. Description of the Related Art
In general, an existing code division multiple access (CDMA) mobile communication system is based on the IS-95 standard which mainly supports a voice service. However, in the near future, mobile communication will be performed in accordance with the IMT-2000 standard which supports not only the voice service but also a high-speed data transfer service. For example, the IMT-2000 standard can support a high-quality voice service, a moving picture service, an Internet search service, etc.
In the CDMA mobile communication system, a radio link between a base station and a mobile station is divided into a forward link for transmitting a signal from the base station to the mobile station and a reverse link for transmitting a signal from the mobile station to the base station.
Reverse (link) common channels include a reverse common control channel (R-CCCH) and a reverse access channel (R-ACH). The reverse common control channel and the reverse access channel undergo power adjustment by the exchange of a message and an acknowledge signal (ACK) between a base station and a mobile station, but do not undergo fast closed-loop power control which will be described below.
A description will be made hereinbelow regarding a conventional power control method for a reverse common control channel and a reverse access channel.
The power control method is divided into an open-loop power control method and a closed-loop power control method. In the open-loop power control method, a mobile station measures the strength of a signal received over the forward link and compares the measured value with a threshold. When the strength of the received signal is smaller than the threshold, the mobile station increases transmission power; otherwise, when the strength of the received signal is greater than the threshold, the mobile station decreases the transmission power.
In the closed-loop power control method, a mobile station transmits a message to a base station over a reverse link and then determines whether or not an acknowledge signal (ACK) is received from the base station. Upon failure to receive the acknowledge signal, the mobile station transmits the message again with transmission power increased by a predetermined level so as to access the base station.
The reverse common control channel and the reverse access channel are contention-based channels. When multiple mobile stations simultaneously attempt to access a base station through the channels, contention may occur between them. In this case, the respective mobile stations again attempt to access the base station with increased power, causing an increase in interference between the reverse links of the mobile communication system. This results in performance degradation of other mobile station""s channel qualities. This problem will become clearer in the following descriptions.
FIG. 1 illustrates a conventional power control method for the reverse link common channel. Referring to FIG. 1, a mobile station sends a preamble signal to a base station with minimum power in an attempt to access a base station. In FIG. 1, T1 denotes the time when the first attempt to access the base station is made. When it fails to access the base station because of contention with other mobile stations, the mobile station waits for a predetermined time, and then, at time T2, again attempts to access the base station by increasing the transmission power. When it fails to access the base station again, the mobile station waits for a predetermined time again, and then, at time T3 attempts to access the base station with a further increase in transmission power. Here, the predetermined time is the sum of a fixed time and a random time.
In this case, contention occurring among multiple mobile stations may cause an increase in interference of the overall system due to the overly increased transmission power. Therefore, when packet data is transmitted over a reverse common control channel, this excessive transmission power problem may occur. Accordingly, there is a need for a fast power control method that suppresses excessive transmission power.
In general, a power control method for the reverse traffic channel aims at maintaining a signal-to-noise ratio (Eb/No) for a signal received from a base station. However, in a power control method for the reverse common channel, multiple mobile stations are controlled by one power control command stream, thus reducing the excessive reverse transmission power.
It is an object of the present invention to provide a power control device and method for reducing the excessive signal power of a reverse common channel in a mobile communication system.
It is another object of the present invention to provide a device and method for controlling transmission power of a reverse common channel using both open-loop power control and closed-loop power control in a mobile communication system.
To achieve the above and other objects, there is provided a method for controlling the transmission power of a reverse common channel for a mobile station in a CDMA communication system. In this method, the mobile station transmits a preamble signal at predetermined intervals, increasing the transmission power each time, until receipt of an acknowledge signal from a base station, at which time the mobile station accesses the reverse common channel. The method comprises the following steps: measuring the strength of a received signal to generate a first transmission power control signal for the preamble signal; accumulating power control commands received for a predetermined time over a forward common channel to generate an accumulated value, comparing the accumulated value with a threshold, and generating a second transmission power control signal for decreasing transmission power when it is required to decrease the transmission power, and maintaining a present transmission power when it is required to increase the transmission power; and accumulating the first and second transmission power control signals and controlling the preamble signal according to the accumulated transmission power control signal.